1. Field of the Invention
The present invention relates to electrical circuits for providing a current through a load impedance where the value of that current is a function of an input voltage and is substantially independent of the value of the load impedance.
2. Description of the Related Art
Several types of current sources have been designed in the past. One type of current source is known as a "Holland" current source. The disadvantage of a Holland current source is that it requires precision resistors that track each other over temperature. Motorola publication AN587 "Analysis And Design Of The Operational Amplifier Current Source", 1973, pp 1-7, provides a detailed analysis of this type of current source. FIG. 1 shows the schematic for a Holland current source. For this type current source, the output impedance is given by equation 1. ##EQU1##
Since the output impedance of a current source should be infinite, this equation illustrates that the ratio of R3 to R4 must be matched to the ratio of R2 to R1. It is this matching that requires the precision resistors to track each other over temperature. Since it is very difficult to maintain this match, the output impedance of this current source is reduced which results in the output current beginning to vary as a function of the load impedance.
Another type of current source uses a floating reference and a grounded load. An example of this type of current source can be found in EDN article "Op Amps Turn Voltage References Into Current Sources", Apr. 26, 1990, Pages 191 to 198. FIG. 2 illustrates one of the circuits discussed in this article. In this circuit, the floating reference is the zener diode. As can be seen in the Figure, the reference and current source are grounded through the load impedance.
JFET 10 biases zener diode 12 to establish a reference voltage Vr between inverting operational amplifier terminal 14 and operational amplifier output 16. As a result, the reference voltage also appears between noninverting operational amplifier terminal 18 and output terminal 16. This results in the reference voltage appearing across resistor 20. Since the polarity of the reference voltage is such that operational amplifier terminal 18 is more positive than operational amplifier terminal 16, this circuit will act as a current sink, and sink a current which is substantially equal to the value of the reference voltage divided by the value of resistor 20.
The floating reference current source has the disadvantage of using a zener diode which is sensitive to temperature variations. It should also be noted that this circuit has the added disadvantage of only being able to sink a current; if it is desirable to have this circuit act as a current source, the JFET and zener diode should be reversed. Yet another disadvantage is that changing the compliance or voltage available to the load, requires changing the supply voltage.